We are sitting in a circle. Incense drifts across the room. A beaming young woman tells us that egg production is up. Everyone smiles. We celebrate the first grapes from the vineyard with some juice. I gulp it down. At the bottom of my cup there is a scrawled message: "Help me get out …" All is not as benign as it seems in Something Got a Hold of Me, an intriguing, sinister new piece of work about religious cults from the Special Guests.

Meanwhile, in the basement, another kind of brainwashing is taking place. Guru Guru – the latest piece from Rotozaza, who had a worldwide hit with Etiquette in 2007 – is a performance for five people at a time. Wearing headphones, we are told what to say and think as members of a focus group who become increasingly in thrall to a deranged on-screen virtual presence. Many have found this exercise in control disturbing, and you may find yourself frantically looking for yourself again in the moments after the performance has finished.

Yes, it's just another typical day at Forest Fringe, the free venue on Bristo Place now in its third year of wildly ambitious experiment. Wander in and you are guaranteed to be surprised. Last week I caught the thrilling and desperately moving From Where I'm Standing, created by Glasgow teenagers and their parents, and watched H Plewis's Anti-Burlesque Burlesque T-shirt dance, a short, sharp statement about the South African HIV epidemic. Much of the work at Forest Fringe is fragile, small and participatory; it tends to reflect something unfinished or broken, and it punches way above its weight.

Upstairs in Bristo Hall, passers-by are being asked to contribute to Jo Bannon's interactive project, Claim to Fame, which builds a community of strangers based on their encounters with famous people. And in the hallway, a crowd has gathered to watch another group of artists, Action Hero, try out something that may find its way into their new show. Afterwards, we eagerly answer questions – thrilled to be part of the process, not just watching a product.

For Peter McMaster's House, the audience are given a guided tour of a home, then encouraged to smash up the furniture and recreate it in more utopian mode, working together to create something optimistic from the broken shards. Then there's Little Bulb, who each night are presenting Sporadical, a work in progress: it's a folk opera featuring a prostitute-turned-mermaid and sea shanties.

Like the whole of Forest Fringe, it is boundlessly enthusiastic, insanely optimistic, entirely free and inexpressibly charming.